The present invention relates to the field of underwater sonar equipment, and more particularly, to shallow water sonar.
Existing sonar systems effectively detect targets in deep water, but are less effective against targets immersed in shallow water. Prior to the present invention, it had been difficult to cope with the reverberation and multipath returns found in shallow water. Reverberation is a narrow bandwidth, noise-like signal from a variety of sources in the undersea environment, such as temperature and density imhomogeneities in the sea, marine life, rough features of the ocean bottom, and reflecting facets and bubbles at the ocean surface. Reverberation interferes with the acoustic echo from a submerged target, particularly in the case of slowly moving targets. Since the sources of reverberation are slowly moving as well, the doppler frequency shift associated with echoes from the reverberation sources are comparable to those from the target, weakening the discrimination between target and reverberation. Reverberation interferes with the acoustic echo from a submerged target.
These problems restrict detection range, heretofore requiring the use of more equipment to cover a given search area. Other problems encountered with conventional active sonar systems are, severe propagation losses from multiple encounters with the ocean surface and bottom; and loss of signal coherence due to the multipath nature of the propagation in shallow water.